Mode attenuating support bead for a coaxial transmission line



Dec. 1, 1970 c, n- 3,544,928

MODE ATTENUATIXG SUPPORT BEAU FOR. A COAXIAL 'IRANSMISSION LINE Filed March 15, 1968 RESISTIVE FILM FOR ATTENUATING CIRCUM- FERENTIAL WAVEGUIDE MODE-l8 9iure 1 Fiure 2 INVENTOR RICHARD C. KEITER ATTORNEY United States Patent US. Cl. 333-97 2 Claims ABSTRACT OF THE DISCLOSURE A center conductor is coaxially supported within an outer conductor by a dielectric support bead having an annular resistive film to attenuate circumferential waveguide modes.

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates to coaxial transmission lines and, more particularly, to support beads for use therewith.

In air-filled coaxial transmission lines the center con ductor is typically supported within the outer conductor by annular dielectric support beads. These dielectric support beads lower the frequency at which the coaxial transmission line supports undesirable circumferential waveguide modes. Moreover, they resonate in the presence of the circumferential waveguide modes. This causes insertion loss and may also cause undesirable coupling between normally isolated parts of the coaxial transmission line. In any case, circuferential waveguide modes in the dielectric support beads may seriously degrade the useful frequency range of the coaxial transmission line.

Accordingly, it is the principal object of this invention to attenuate circumferential waveguide modes in the dielectric support beads and thereby extend the useful frequency range of the coaxial transmission line.

This object is accomplished according to the illustrated embodiments of this invention by providing a coaxial transmission line with a dielectric support bead having an annular resistive film in a region where a circumferential waveguide mode has a finite electric field.

Other and incidental objects of this invention will be apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIG. 1 is a sectional perspective view of a coaxial transmission line according to one embodiment of this invention; and

FIG. 2 is a sectional side view of a coaxial transmission line according to another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown an air-filled coaxial transmission line including a center conductor 10 coaxially supported within and spaced from an outer conductor 12 by an annular dielectric support bead 14. The outer diameter of center conductor 10 is stepped down and the inner diameter of outer conductor 12 is stepped up at dielectric support bead 14, as best illustrated in the sectional view of FIG. 2, to maintain a constant characteristic impedance along the transmission line. An annular channel 16 is formed in one or both sides of dielectric support bead 14 between conductors 10 and 12 to inductively compensate for the change in capac itance caused by these steps in conductors 10 and 12. Circumferential waveguide modes in dielectric support bead 14 are attenuated by an annular resistive film 18 formed in channel 16 at a position where the circumferential waveguide modes have a finite electric field. Resistive film 18 thereby presents a loss to the circumferential waveguide mode causing at least some of its energy to be spent as heat. Although an annular resistive film 18 is shown, any resistive film positioned on at least one side of dielectric support bead 14 in a region where a circumferential waveguide mode has a finite electric field helps to attenuate that circumferential waveguide mode. A fifty to four hundred ohm per square resistive film, as measured on glass, may be used. The film should be made as small as possible and positioned to present the most loss to the waveguide mode with the smallest possible loss to the normal TEM mode of the coaxial transmission line. The resistive film should not contact conductor 10 and 12.

Referring now to FIG. 2, there is shown another airfilled coaxial transmission line in which support head 14 comprises two concentric annular sections 18 and 20. A11 annular resistive film 22 is formed on the inner periphery of support bead section 18 and/ or on the outer periphery of support bead section 20 in a region where a circumferential waveguide mode to be attenuated has a finite electric field. This structure maximizes the loss presented to the circumferential waveguide mode while at the same time minimizing the loss presented to the normal TEM mode of the coaxial transmission line. In addition, the resistive film of this structure does not appreciably alter the reflection coeflicient of dielectric support bead 14.

What is claimed is:

1. Coaxial transmission line apparatus comprising:

an outer conductor;

an inner conductor;

a dielectric element coaxially supporting said inner conductor within said outer conductor; and

an annular resistive layer electrically insulated from said conductors and supported on one side of said dielectric element in a region where a circumferential waveguide mode in said dielectric element has a finite electric field to attenuate said circuferential waveguide mode.

2. Coaxial transmission line apparatus comprising:

an outer conductor;

an inner conductor;

a dielectric element coaxially supporting said inner conductor within said outer conductor, said dielectric element comprising a pair of concentric sections; and

an annular resistive layer supported on a peripheral portion of one of said concentric sections along an annular interface therebetween in a region where a circumferential waveguide mode in said dielectric element has a finite electric field to attenuate said circumferential waveguide mode, said resistive layer being electrically insulated from said conductors.

References Cited UNITED STATES PATENTS 2,434,560 l/1948 Gunter 338-216X 2,946,966 7/1960 Crowe 3338l(A)X 3,008,102 11/1961 St. Clair 333-98(M)X OTHER REFERENCES T. Moreno, Microwave Transmission Design Data, Dover Pub., N.Y., 1948, p. 69 relied on.

PAUL L. GENSLER, Primary Examiner US. Cl. X.R. 333-98 

